Moving Qa ldmatrix into the tile loop caused warps 0-2 to read
warp 3's Q data from sQ (only the last warp's data survives the
serialized load loop). Reverted to loading Qa during the init phase
and keeping it resident; __launch_bounds__ still forces 128 regs
(33% occupancy) with spill to local memory.
ViperEkura
dea59f7e1d